1. Field of the Invention
The present invention relates to a DC-DC converter of a switching regulator system to convert a direct-current voltage, and for example, to a technique which is effective for being applied to a DC-DC converter comprising a pulse-width modulation (PWM) control mode and a pulse-frequency modulation (PFM) control mode, to perform a PFM drive when a current flowing in a load becomes small in amount
2. Description of Related Art
There is a DC-DC converter of a switching regulator system as a circuit to convert an input direct-current voltage so as to output a direct-current voltage having a different potential. As such a DC-DC converter, there is a DC-DC converter comprising a drive switching element to apply a direct-current voltage supplied from a direct-current power source such as a battery, and the like, to an inductor (a coil) and to flow the input current so that an energy is accumulated in the coil; a rectification element to rectify the current in the coil during an energy discharging period at which the drive switching element is set to be off; and a control circuit to control the on and off of the drive switching element.
In such a DC-DC converter of the switching regulator system, the following control is generally performed. That is, the output voltage is detected by an error amplifier so that the detected output voltage is performed with feedback to a pulse-width modulation (PWM) comparator or a pulse-frequency modulation (PFM) comparator. Further, the time while the switching element is set to on is prolonged when the output voltage decreases, and the time while the switching element is set to on is shortened when the output voltage increases.
Further, in the PWM control, a pulse width is fluctuated according to the load, in a state where the period (the frequency) of the drive pulse is maintained to be even. Even when the load is very light, the pulse having the minimum pulse width which is determined by the characteristics of the circuit is to be output. Further, a case may occur in which when the fluctuation width of the load is very large, the output current is to be too much even when driven by the pulse having the minimum pulse width. Accordingly, there also is a DC-DC converter comprising the PWM comparator and the PFM comparator. In such a DC-DC converter, the PWM control is performed under a normal condition, and in a case where the current flowing in the load decreases, that is to say, when the load is light, the converter is driven by the pulse width of an even fixed pulse, and the frequency is fluctuated according to the load, thereby the converter is shifted to the PFM control. Such an invention pertaining to the DC-DC converter is disclosed, for example, in Japanese Patent Application Laid-open Publication Nos. 2006-149067 and 2003-219637.
In a portable electronic device such as a mobile phone, when the DC-DC converter in which the above described PWM control and the PFM control are performed by being switched to and from each other, is applied as a power source equipment, the device is operated in the PFM control at a standby state in which a main body section and a display section are folded up, because the electric power consumption is very small in amount. When the display section is opened from this state, the electric power consumption is increased by some degree due to the lighting of the display section, and the like. However, the electric power consumption is not increased on a grand scale suddenly. Still, the electric power consumption may likely be increased when the subsequent operations are to be performed. Thus, it is preferable for the power source equipment of the device to previously be shifted to the PWM control mode from the PFM control mode, so as to be capable of accepting such operations quickly.
In the PFM control, the pulse frequency thereof is longer than that of the PWM control, however, the pulse width thereof is broader than that of the PWM control. Thus, the output of the error amplifier to detect the output voltage is to fluctuate greatly. Therefore, when the PFM control is switched to the PWM control in the above described manners it is discovered that there is a possibility that the output voltage may be greatly decreased, depending on the timing of the switching.
To put it concretely, as shown in FIGS. 4A-4E, for example, when the switching from the PFM to the PWM is performed in a relatively quick timing (time t1) from the output of the drive pulse in the PFM control, the output Verror of the error amplifier is not sufficiently raised. Thus, in a case of the control to start the PWM control pulse, at the timing where the waveform signal (a triangular wave) RAMP to be input to the PWM comparator reaches the output level of the error amplifier, or intersects the output thereof, the PWM control pulse may significantly be narrowed than the PFM control pulse. Therefore, the smoothing condenser at the output side cannot be supplied with a sufficient charge, the output voltage Vout is greatly decreased as shown in FIG. 4C, and the fluctuation of the output voltage Vout is to be large